14.4 Hurricane Helene (2006): A case of Saharan Air Layer influence?

Wednesday, 19 August 2009: 4:45 PM
The Canyons (Sheraton Salt Lake City Hotel)
Scott Braun, NASA/GSFC, Greenbelt, MD; and C. L. Shie, J. Sippel, and D. S. Nolan

The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impact on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) low-level vertical wind shear associated with the African easterly jet; 2) warm air aloft, which increases thermodynamic stability; and 3) dry air, which produces cold downdrafts. Some investigators have assumed the validity of these proposed negative influences and have frequently used them to explain the failure of individual storms to intensify or to explain the relative inactivity of recent hurricane seasons. Multiple NASA satellite data sets and National Centers for Environmental Prediction global analyses are used to characterize the SAL's properties and evolution in relation to the development and evolution of Hurricane Helene (2006). The results will show that neither jet-induced vertical wind shear nor warm SAL air (high stability) were likely to have produced significant negative impacts on Helene. Dry air is readily evident in the storm's environment, but was it a key mechanism for SAL influence? Idealized simulations will be used to evaluate the role of dry air.
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